#ifdef GL_ES
precision mediump float;
#endif

#define PI 3.14159265359

uniform vec2 u_resolution;
uniform float u_time;

mat2 rotate2d(float _angle){
    return mat2(cos(_angle), -sin(_angle), sin(_angle), cos(_angle));
}

float box(in vec2 _st, in vec2 _size){
    _size = vec2(0.5) - _size*0.5;
    vec2 uv = smoothstep(_size, _size+vec2(0.001), _st);
    uv *= smoothstep(_size, _size+vec2(0.001), vec2(1.0)-_st);
    return uv.x*uv.y;
}

float cross(in vec2 _st, float _size){
    return box(_st, vec2(_size, _size/4.0)) + box(_st, vec2(_size/4.0, _size));
}

void main(){
    vec2 st = gl_FragCoord.xy/u_resolution.xy;
    vec3 color = vec3(0.0);

    st -= vec2(0.5);
    st = rotate2d(sin(u_time)*PI) * st;
    st += vec2(0.5);

    color = vec3(st.x, st.y, 0.0);
    color += vec3(cross(st, 0.4));

    gl_FragColor = vec4(color, 1.0);
}
